In the drilling, completion, and stimulation of oil and gas wells, well treatment fluids are often pumped into well bore holes under high pressure and at high flow rates causing the rock formation surrounding the well bore to fracture. A type of well treatment commonly utilized for stimulating hydrocarbon production from a subterranean zone penetrated by a well bore is hydraulic fracturing. Hydraulic fracturing, also referred to as fracing (or fracking), is used to initiate production in low-permeability reservoirs and re-stimulate production in older producing wells. In hydraulic fracing, a fluid composition is injected into the well at pressures effective to cause fractures in the surrounding rock formation. Fracing is used both to open up fractures already present in the formation and create new fractures. Proppants, such as sand and ceramics, are used to keep induced fractures open both during and after fracturing treatment. To place the proppants inside the fracture, the proppant particles are suspended in a fluid that is pumped into the subterranean formation. Generally, this fluid has a viscosity sufficient to maintain suspension of the particles.
For ideal performance, a hydraulic fracturing fluid should be sufficiently viscous to create a fracture of adequate width and be able to transport large quantities of proppants into the fracture. The viscosity of the fluid can be enhanced or modified by addition of synthetic and/or natural polymers, or other rheology modifiers. Examples of polymer-enhanced fluids used to increase the viscosity of hydraulic fracturing fluids include slickwater systems, linear gel systems, and crosslinked gel systems. Of these, crosslinked gel systems are the most viscous.
In a crosslinked gel system, a linear polymer or gel, for example, a fluid based on guar or modified guar, is crosslinked with added reagents such as borate, zirconate, and titanate in the presence of alkali. The most common version of crosslinked gel is known in the art as guar-borate gel. The crosslinked gel fluid increases the viscosity of the fracturing fluid, such that proppants can be effectively suspended.
Once the hydraulic fracturing fluid has delivered proppant to the fracture or delivered sand in gravel packing or frac packing operations, it is often desirable to lower the viscosity of the fracturing fluid such that the fluid can be recovered from the formation using minimal energy. The removal of the spent fracturing fluids from the subterranean formation is typically required to allow hydrocarbon production. This reduction in viscosity of the fracturing fluid is often achieved using a breaker, i.e., a compound that breaks the cross-linking bonds within the gel.
Synthetic polymers, for example polyacrylamide (PAM) polymers, can form permanent gels under acidic conditions with metal crosslinking agents, such as aluminum-, chromium-, zirconium- and titianium-based complexes. Such gels can be used, for example, to control conformance in enhanced oil recovery (EOR) applications, where subsequent breaking to significantly reduce viscosity is not necessary. However, for fracing fluid applications, the acidity of the formation in hydraulic fracturing is usually not high, and breaking of the crosslinked gel improves fluid recovery.